The invention is directed to a phase mask for projection lithography with light having a wavelength .lambda. for use in an exposure means having an imaging scale m and having a numerical aperture NA. The invention is also directed to a manufacturing method for such a phase mask.
Higher and higher packing densities are desired in the development of semiconductor modules. This requires a greater and greater reduction in the minimal dimensions of the individual elements. Structural finenesses of less than 1 .mu.m have already been achieved with the assistance of modern semiconductor technologies.
Constantly increasing demands made of the structuring technique are involved with the constant reduction in the minimum dimensions of the individual elements. In particular, the resolution of the exposure means utilized for the photolithography must be further improved in order to permit structuring that is dimensionally true.
The resolution is limited by diffraction effects at structural edges of a projection mask employed in the exposure means. When the light passes through the projection mask, a part of the passing luminesce intensity--as a result of diffraction effects--proceeds into regions that are covered by the projection mask.
Employing what is referred to as a phase-shifting mask or phase mask for the reduction of diffraction effects is known from M. D. Levenson et al., IEEE ED-29 (1982), page 1828. A phase mask is a projection mask wherein light that has passed through neighboring openings of the projection mask is shifted in phase. In this known phase mask, the phase shift amounts to 180.degree.. A destructive interference between the two openings thereby arises, given exposure with coherent or partially coherent light. What this affects is that the intensity between the two openings is minimized. A phase mask is realized in that an opening in the projection mask is provided with a light-transmissive layer having the thickness d=.lambda./(2 (n-1)), where n is the refractive index of the light-transmissive layer, and .lambda. is the wavelength of the light.
In the manufacture of the phase mask, a layer of electron beam lacquer is applied onto a carrier that is provided with a mask pattern of light-absorbent material. This electron beam lacquer is structured in the above-described way with the assistance of electron beam lithography. An alternative is to apply a layer of SiO.sub.x or MgF.sub.2 onto the finished mask pattern, this layer being structured with the assistance of electron beam lithography and subsequent dry-etching.
T. Terasawa et al., Proc. SPIE 1088 (1989), page 25 discloses a phase mask wherein electron beam lacquer is likewise employed in order to produce phase-shifting regions on the mask.
I. Hanyu et al., Abstract SPIE 1264 (1990) discloses a phase mask wherein the phase-shifting regions are composed of SiO.sub.2. For manufacture, an electron beam lacquer structure is produced on the finished mask with the assistance of electron beam lithography. A SiO.sub.2 layer is applied onto this electron beam lacquer structure. The regions of the SiO.sub.2 having electron beam lacquer lying under them are removed with a lift-off process.
M. Nakase et al., Preprint IEDM (1989) discloses a phase mask wherein the phase-shifting regions are composed of electron beam lacquer. The phase-shifting regions are arranged in self-aligned fashion on the light-absorbing mask pattern. The electron beam lacquer projects beyond the light-absorbing regions on the mask at the respective edges thereof. As a result thereof, the light-transmissive regions each receive a respective border that shifts the light phase by 180.degree.. The work by Prouty et al., Proc. SPIE 470 (1984), page 228 discloses that a reduction of the diffration effects can already be achieved with phase masks that effect a phase difference of 180.degree..+-.60.degree..
What the methods for manufacturing phase masks of Levenson et al., Terasawa et al. and Hanyu et al. have in common is that they each require two electron beam lithography processes. This makes complicated equipment necessary for producing the masks. The method of Nakase et al. has the disadvantage that it employs masks having phase-shifting regions of electron beam lacquer. Such masks are difficult or impossible to clean. Moreover, the lacquer absorbs in the deep UV and, over and above this, has a refractive index differing from that of the mask carrier, this leading to multiple interferences.